Ultra-high purity gases are widely used in the semiconductor industry and must meet stringent particulate content specifications. In order to assure that a gas supply meets such specifications, a sampling system is connected to the gas supply and provides samples of the gas to a particle analyzer. The sampling system analyzer must generally perform the following functions:
(1) provide a sample from the pressurized process gas stream that is isokinetic with the gas stream;
(2) reduce the pressure of the process gas sample to atmospheric;
(3) transport the atmospheric level sample to the particle analyzer; and
(4) vent analyzed gases safely from the system.
As used herein, the term "isokinetic" means that gas entering the sampling system exhibits equal velocity and kinetic energy to the gas flow in the pressurized process gas stream. Such isokinetic sampling is generally achieved by assuring that the probe which samples the gas is oriented along the flow path and is positioned parallel to the gas flow from which the sample is taken. The sample gas flow is adjusted through, for example, use of a restrictor.
Recent advances in particle analyzers have extended their use to process gases other than air or nitrogen. For instance, condensation nucleus particle counters (CNC's) have been introduced that are usable in oxygen and hydrogen gas streams. Such a CNC particle counter (i.e. the CPC 7651) is available from TSI Inc., 500 Cardigan Rd., St. Paul, Minn. 55164. CNC type instruments require that the gas sample be supplied at slightly above atmospheric pressure to force the sample through the counter; that a source of purge gas be provided so as to enable purging from the CNC of the reactive gas; and that a liquid fill system be provided which supplies the instrument with a working fluid. Such a liquid fill system requires slight pressurization to overcome the sample gas pressurization level.
Borkman et al. in "Providing Next-Generation Particle Measurement and Control For Ultra-High-Purity Gas Distribution Systems" Microcontamination, March 1992, disclose a gas sampling system that is usable with ultra high purity gas sources. Borkman et al. employ a pitot probe positioned iso-axially in the process gas line. Typical process gas pressures are on the order of 80 to 120 psig. The flow withdrawn by the pitot probe and its diameter are adjusted to allow iso-kinetic sampling of the process gas from the pipeline. The pitot probe is connected to a pressure reduction device which reduces the sampled gas to atmospheric pressure. Details of the pressure reduction device are disclosed in U.S. Pat. No. 4,998,954 to Burr, the disclosure of which is incorporated herein by reference. Briefly stated, Burr describes a retractable iso-kinetic probe that is insertable into the gas flow line through an aligned guide tube. The probe provides an iso-kinetic gas sample to a converging/diverging nozzle where the pressure of the gas is reduced to atmospheric by a controlled shock wave.
The Burr probe and pressure reduction assembly neither loses particles in the sample stream nor releases additional particles to it. After pressure reduction, the sampled gas is transported to a sample horn where a second pitot probe draws the actual sample that is to be analyzed by the particle counter. Again, the second pitot probe diameter is adjusted to provide iso-kinetic sampling of the approximately atmospheric level gas sample.
At least two types of counters have been employed in prior art gas sampling systems. The first type comprises laser-based particle counters which count and size individual particles directly. Such a counter may be used directly at process gas line pressure and also may be used with hydrogen and oxygen directly. The limitation of laser-based particle counters is that the lowest detectable particle size is on the order of 0.05 to 0.1 microns in diameter. A second type of particle counter is the CNC type, described above. Until recently, CNC type counters have been restricted to use at atmospheric pressure and in air or with an inert gas. CNC counters, however, offer detection limits of less than 0.01 microns and thus, in theory, they can provide lower detection limits by a factor of 5 to 10 over laser-based counters. Present CNC designs employ an internal pump, or a critical orifice with an external sample pump, to assure a flow of sample gas to the CNC. Such pumps render the CNC type counter unusable with hydrogen or oxygen gas because the pump is a potential ignition source.
U.S. Pat. No. 5,231,865 to McDermott et al. discloses a method for use of a CNC counter in a hydrogen gas flow. McDermott et al. utilize a dilution and diffusion apparatus positioned ahead of the CNC. Their approach is to convert the processed hydrogen sample stream from pure hydrogen, which the particle counter cannot handle, to a mixture of hydrogen and predominately nitrogen, which can be handled safely by the instrument.
Although the dilution method disclosed by McDermott et al. is useful for sampling hydrogen, it has a number of disadvantages. First, the diffusion/dilution apparatus is mechanically complex and gas flow rates and flow patterns must be carefully maintained. Next, the apparatus places another device in the sample flow path between the sample point and the CNC counter, which device may itself generate particles resulting in an erroneously high particle concentration reading by the CNC. Furthermore, the transport losses of particles in the device may lead to an erroneously low particle count. Lastly, because the CNC is used in a mixture of hydrogen and nitrogen, there is a question whether or not the particle counter is performing as accurately in the hydrogen/nitrogen mixture as it does in a pure hydrogen flow.
Accordingly, it is an object of this invention to provide a particle sampling system with means for providing a sample gas at an overpressure to a particle analyzer.
It is another object of this invention to provide a particle sampling system wherein a means for providing the sample gas at an overpressure to a particle analyzer provides sample gas which is representative of the pressurized gas.
It is a further object of this invention to provide a particle sampling system which is usable with reactive gases.
It is yet another object of this invention to provide a particle sampling system wherein provision is made to provide a working fluid to the particle analyzer when the sample gas is supplied at a slight overpressure.